Sensorless closed-loop actuator unlatch

ABSTRACT

A first method for detecting movement of an actuator includes supplying a demand current to a voice coil motor of the actuator (A 1 ), sampling a voice coil motor voltage (B 1 ), performing a slope detection of a voice coil motor voltage function (C 1 ) and extrapolating the voice coil motor function (D 1 ) to identify a change in slope of the voice coil motor function. 
     A second method for detecting movement of an actuator includes supplying a continuously increasing current ramp to the voice coil motor of an actuator (A 2 ), measuring a back e.m.f. voltage of the voice coil motor (B 2 ), performing a change in slope detection of a back e.m.f. voltage function (C 2 ) and integrating the back e.m.f. voltage function (D 2 ) to determine movement of the actuator.

BACKGROUND OF THE INVENTION

The present invention relates to controlling an actuator in a disc drivesystem. More particularly, the present invention relates to accuratelydetermining the time of actuator unlatch in a disc drive.

In a disc drive system, it is sometimes desirable to restrain theactuator from movement. For example, transporting the disc drive orperiods during which the disc drive is not in operation require that theactuator be restrained or "latched". An actuator has attached to itshead-arm assembly, multiple magnetic read/write heads which are delicateinstruments. Therefore, if the actuator is allowed to move when thedrive sustains rough handling, such as during shipping, damage to theheads and/or the disc could occur.

The actuator may be restrained using a magnetic latch which holds theactuator in place over a specific area on the magnetic media disc. Onzone bit recording drives, the initial demand current pulse whichunlatches the actuator also accelerates the actuator. Immediately afterunlatch, the actuator is moved to the outer zone of the disc tosynchronize with the servo pattern of the disc. The initial accelerationdemand current pulse is followed by an opposite deceleration pulse toslow the head to zero velocity to retrieve the servo pattern. Track seekoperations begin after the servo pattern has been retrieved to move theactuator to specific disc locations.

To unlatch the actuator and accelerate it to a desired location on amagnetic media disc, a transconductance amplifier circuit converts inputvoltage, originating from the controlling code in the CPU, to demandcurrent. The demand current is supplied to a voice coil motor (VCM) ofthe actuator. The initial demand current unlatches the actuator (intheory) and accelerates it to an outermost zone of the disc so that theservo pattern of the disc may be retrieved. After the actuator isunlatched and it begins to move, however, a "back" electromotive force(e.m.f.) voltage appears across the VCM. Thus, when the actuator is inmotion, such as when it is enroute to the outermost zone of the disc,the transconductance amplifier circuit supplies voltage to the VCM tocompensate for the back e.m.f. voltage.

The existing unlatch/track seek process is essentially an open loopprocedure. As a result, there is no means for the controlling code toreceive an indication that the actuator has actually been unlatchedafter the initial demand current pulse. If the first unlatch attemptfails, the entire process must be repeated with stronger and longerduration current pulses until the actuator is unlatched. Increasing thedemand current, however, may cause the magnetic read/write head tocatastrophically crash into the magnetic media disc. Damage to themagnetic head will occur if the latch is actually a weaker latch thananticipated and an excessive acceleration demand current has beenapplied.

Variations in manufacturing tolerances associated with the components ofthe latch (i.e., magnetic pole pieces, rubber housing etc.) haveresulted in magnetic latches with poorly controlled latch forces.Present open-loop unlatch procedures often cannot unlatch strong latcheson the first attempt. Poorly controlled latch forces have caused asignificant problem with the conventional unlatch and accelerationprocess in that it is impossible to determine how much of the initialdemand current was used to unlatch the actuator and how much resulted inacceleration. Thus, it is difficult to estimate the amount ofdeceleration current necessary to guarantee zero velocity at the end ofacceleration.

SUMMARY OF THE INVENTION

The present invention uses the back e.m.f. voltage of the VCM todetermine the time at which an actuator actually unlatches from themagnetic latch. More particularly, if the actuator is stationary (i.e.latched) there will be no back e.m.f. voltage. Once the actuator isunlatched and moves, however, a back e.m.f. voltage will be induced. Bydetermining the point in time at which the back e.m.f. voltage appears(or changes), the time of actuator unlatch can be ascertained.

A first method employs an algorithm which detects the presence of aslope in a "VCM-HI" voltage function. By sampling the voltage at theVCM-HI node of the transconductance amplifier circuit, the back e.m.f.voltage can be indirectly obtained. More particularly, if the actuatoris latched, the VCM-HI voltage will be a constant value (i.e. zeroslope). Once unlatch occurs, however, the VCM-HI voltage will change tocompensate for the change in back e.m.f. voltage. The voltage at theVCM-HI node is sampled to construct a VCM-HI voltage function. When aslope change is detected on the VCM-HI voltage function, actuatorunlatch has occurred. Once unlatch is determined, the VCM-HI voltagefunction is extrapolated to determine actuator time of unlatch.

A second method directly measures the back e.m.f. voltage generated bythe VCM. Using the second method, however, requires a supplementalcircuit. The algorithm of the second method requires that a rampeddemand current be supplied to the VCM. As the incrementing demandcurrent is supplied to the VCM, the back e.m.f. voltage is measuredusing the supplemental circuit. A change in slope of the back e.m.f.voltage function indicates unlatch of the actuator. Once unlatch isdetermined, the back e.m.f. voltage function is integrated to determinethe movement of the actuator.

In addition to providing confirmation of actuator unlatch, both methodsprovide actuator movement information which may be used to determine therequired deceleration demand current needed to stop the actuator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a drive circuit which provides demand current to the VCM ofthe actuator.

FIG. 2 is a flow diagram of the steps of a first method of the presentinvention.

FIG. 3 shows plots of demand current and the VCM-HI voltage function forsuccessful unlatch using the first method.

FIG. 4 is a flow diagram of the steps of a second method of the presentinvention.

FIG. 5 shows plots of demand current and the back e.m.f. voltagefunction for successful unlatch using the second method.

FIG. 6 shows the added circuitry used to measure back e.m.f. voltage inthe second method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a typical bi-directional drive transconductance poweramplifier circuit 10 used to drive a VCM. Circuit 10 includes poweramplifiers 14 and 16 arranged in a push-pull configuration with currentsense resistor R₅ and the VCM (represented by the series combination ofinductor L₁ and resistor R₁₁). Also shown are resistors R₁, R₂, R₃, R₄,R₇, R₈, R₉ and R₁₀, input voltage V₁, reference voltage V_(ref) andsupply voltage V_(S). The back e.m.f. voltage developed across the VCMis represented by voltage V_(bemf). The connection from transconductanceamplifier circuit 10 to the VCM is denoted as VCM-HI and VCM-LO at nodesN₁ and N₂, respectively.

In operation, current I_(O), through the VCM and sense resistor R₅ is alinear function of input voltage V_(I) represented by the expression:##EQU1##

For a given input voltage V_(I), transconductance amplifier circuit 10will attempt to maintain the specified output current I_(O).Accordingly, the output voltage across VCM-HI and VCM-LO will change tocorrect for back e.m.f. voltage or VCM resistance change due totemperature variance. Consequently, VCM-HI (and VCM-LO) will be a directfunction of back e.m.f. voltage (V_(bemf)). The VCM-HI voltage value isrepresented by the equation:

    VCM-HI=-V.sub.I (K)+V.sub.S /2-V.sub.bemf /2

where K=R₂ (R₅ +R₁₁)/(2R₁ R₅). When a current is applied to the VCM, avoltage will appear on VCM-HI. The voltage at VCM-HI is due to the firsttwo terms (i.e. -V_(I) (K) and V_(S) /2) of the VCM-HI equation. If theactuator is stationary (i.e. latched) the VCM-HI voltage will remainconstant. When the actuator moves, however, a back e.m.f. voltage(V_(bemf)) will appear as voltage source V_(bemf) causing the VCM-HIvoltage to decrease proportionately. The present invention uses the backe.m.f. voltage to detect movement of the actuator and hence, todetermine the time of actuator unlatch.

FIG. 2 shows a flow diagram of the steps of the first method of thepresent invention.

FIG. 3 shows plots of demand current and the VCM-HI voltage function forsuccessful unlatch using the first method.

In process step A₁, a demand current is supplied to the VCM. Plot 20 ofFIG. 3 shows the acceleration demand current function as it is suppliedto the VCM in accordance with the step A₁. At step B₁, the VCM-HIvoltage at node N₁ and supply voltage V_(S) are sampled (see FIG. 1)using a voltage meter. Sampling continues until a slope change isdetected. Sampling the supply voltage V_(S) serves as a check to ensurethat variations in VCM-HI voltage are not caused by variances in thesupply voltage.

At step C₁, slope detection is performed using present and previoussamples of the VCM-HI voltage. In other words, a VCM-HI voltage functionis plotted using the voltage samples to determine a point on the VCM-HIvoltage function where a slope change occurs.

If after step C₁, a slope change in the VCM-HI voltage function is notdetected, one must determine if the sampling interval cycle has beencompleted. If the sampling cycle is incomplete, sampling must continue.If the cycle has been completed but no slope change was detected, theacceleration demand current is increased in step C_(1A). If byincreasing demand current, a maximum current is reached, other recoveryroutines (e.g. vibrating the actuator to induce unlatch) may be tried.If maximum current is not reached, the sampling interval timer is resetin step C_(1B), and steps B₁ -C₁ are repeated.

Plot 22 of FIG. 3 shows an example of a VCM-HI voltage function. At zeromilliseconds, no current is supplied to the VCM. Accordingly, plot 22shows at zero milliseconds, the VCM-HI voltage is 5.8 volts (i.e. atzero milliseconds VCM-HI is equal to VCM-LO which is half the supplyvoltage V_(S)). At two milliseconds, 200 milliamps of current aresupplied to the VCM. At this point, VCM-HI voltage drops to 3.4 volts.In this case, the VCM-HI voltage is sampled for approximately sixmilliseconds before a slope change occurs at point X₁. Note that atapproximately four, five and seven milliseconds, the acceleration demandcurrent is increased in accordance with step C_(1A). After slopedetection, a deceleration current of -200 mA is applied.

In step D₁, the position of the actuator is extrapolated by noting thetime of first slope detection. By determining the time of unlatch, theposition of the actuator at that time can be determined. A correspondingdeceleration current is then supplied (see plot 20).

At step E₁, actuator position information is output to a controllerresponsible for the actuator acceleration routine.

FIG. 4 shows the flow diagram of the steps of a second method of thepresent invention.

FIG. 5 shows plots of the demand current and the back e.m.f. voltagefunction for successful unlatch of the actuator using the second method.

In process step A₂, a ramped demand current is supplied to the VCM. Instep B₂, the back electromotive force voltage is measured using thecircuitry disclosed in FIG. 6.

In step C₂ a change in slope detection is performed using the backe.m.f. voltage as measured by the circuit shown in FIG. 6. A back e.m.f.voltage function is plotted to determine a point on the function where achange in slope occurs.

Plot 26 of FIG. 5 shows the initial ramped acceleration demand currentprovided to the VCM in step A₂. At zero milliseconds, no current flowsto the VCM. At two milliseconds, the initial acceleration current isapplied to the VCM. The initial current starts at 200 milliamps andincreases incrementally until a change in slope of the back e.m.f.voltage function is detected. After a change in slope is detected, acontrolled acceleration is followed by deceleration.

Plot 28 of FIG. 5 shows the resulting back e.m.f. voltage due to theacceleration demand current supplied to the VCM as shown in plot 26. Inthis case, the back e.m.f. voltage function increases in a constantfashion with current (as expected) for approximately 4 milliseconds. Atabout six milliseconds, however, there is a change in slope of the backe.m.f. voltage function. The change in slope occurs at point X₂.

The algorithm of the second method requires that the acceleration demandcurrent be continuously increased until a change in slope is detected.While the current is supplied to the VCM, the back e.m.f. voltage iscontinuously measured.

If after step C₂, a change in slope is not detected, the initial demandcurrent function is incremented by fixed intervals in step C_(2A). If amaximum current value is attained in step C_(2A), recovery routines areinstigated. If a maximum current value has not been reached, steps B₂-C₂ are repeated.

In step D₂ the back e.m.f. voltage function is integrated to determinethe first detection of the change in slope in order to estimate theposition of the actuator at that time.

In step E₂, the position of the actuator at the time of unlatch isoutput to a controller which is responsible for the acceleration routineof the actuator.

Both first and second methods provide information as to when unlatch ofthe actuator occurs. The position of the actuator at the time of unlatchcan be used to determine the amount of acceleration and/or decelerationcurrent needed to achieve zero velocity of the actuator. By determiningthe actuator position, unnecessary excess current need not be appliedwhich could accelerate the actuator at a rate of speed which may causedamage to the read/write heads and/or the magnetic disc.

FIG. 6 shows VCM back e.m.f. voltage measuring circuit 30 of the presentinvention. Circuit 30 is attached to transconductance amplifier circuit10 (see FIG. 1) in order to directly measure back e.m.f. voltage whenusing the second method.

In a preferred embodiment of the present invention, circuit 30 isattached to circuit 10 at nodes N₁ and N₂. Circuit 30 includesdifferential amplifier 32 and resistors R₁₂, R₁₃, R₁₄, R₁₅, R₁₆ and R₁₇.The voltage across resistors R₁₂ and R₁₃ equals the VCM voltage and theback e.m.f. voltage of the VCM. The back e.m.f. voltage is representedas voltage V_(bemf) in circuit 10. Amplifier 32 isolates the back e.m.f.voltage. Its amplified output, V_(O), is supplied to a CPUanalog-to-digital converter (not shown) so that it may be provided tothe controller.

The invention provides a reliable method of determining when an actuatorunlatch has occurred so that an accurate amount of deceleration currentmay be supplied to the VCM. The method is a cost effective way to savepower and increase efficiency of the unlatch procedures.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A method for detecting movement of an actuatorcomprises:supplying current to a voice coil motor of the actuator;sampling a voice coil motor voltage while supplying current to the voicecoil motor; performing a slope detection on a voice coil motor voltagefunction using the samples; and identifying a change in slope of thevoice coil motor voltage function.
 2. The method of claim 1 and furthercomprising extrapolating the voice coil motor voltage function toidentify when the change in slope of the voice coil motor voltagefunction occurred and providing the change in function information to acontroller.
 3. The method of claim 1 wherein sampling the voice coilmotor voltage comprises attaching a voltage meter to the voice coilmotor.
 4. The method of claim 1 wherein supplying current to the voicecoil motor comprises incrementally increasing the current supplied untila change in the slope of the voice coil motor voltage function isdetected.
 5. The method of claim 1 wherein identifying a change in slopecomprises plotting the voice coil motor voltage function and identifyinga point where a non-zero slope occurs.
 6. A method for detectingmovement of an actuator comprises:supplying a continuously increasingcurrent to a voice coil motor of an actuator; measuring a back e.m.f.voltage of the voice coil motor; performing a change of slope detectionof a back e.m.f. voltage function of voice coil motor; and integratingthe back e.m.f. voltage function to determine movement of the actuator.7. The method of claim 6 wherein measuring the back e.m.f. voltagecomprises attaching a differential amplifier circuit to atransconductance amplifier circuit.
 8. The method of claim 6 whereinperforming a change in slope detection comprises plotting themeasurements of the back e.m.f. voltage.
 9. The method of claim 6wherein supplying a continuously increasing current to the voice coilmotor comprises supplying a ramped current to the voice coil motor untila maximum current is achieved.